1. Field of the Invention
The present invention relates wireless communications systems and, more particularly, to beam-forming technologies and associated methodologies.
2. Description of the Related Art
Antenna array systems with desired beam-patterns have been considered as a solution to improve the spectral efficiency and communication quality for both uplink (mobile-to-base station) and downlink segments (base station-to-mobile) in wireless communication systems. The beam-forming technologies employed with antenna arrays can be a powerful means to increase system capacity, improve quality of service (QoS), reduce co-channel interference (CCI), and multipath fading. Generally, this is because a transmitter/receiver using an antenna array can increase or decrease antenna gain in the intended look directions (i.e., approximate direction of mobile terminal location).
There are several ways to realize such beam-forming technologies. For example, switch beam antenna arrays select a beam pattern out of a set of previously fixed beam patterns, depending on the receiving signal power measurement and spatial location of the desired mobile terminal or base station. Such systems typically comprise multiple antenna elements, a fixed beam-forming network, multiple beam power measurement units, a beam selection unit, and transceiver. For switch beam antenna array, the transmitting/receiving beam is selected by measuring the desired signal power within each beam and selecting the beam having the largest received signal power. The received signal power within each beam may be averaged over the fast fading pattern.
A second example of beam-forming technology is what is employed in dynamically phased array systems. In such systems, the beam pattern is modified based on the look direction of the desired mobile or base station via phase shifter. Dynamically phased array systems typically comprise multiple antenna array elements, multiple phase shifters (one for each antenna element), a weight computation unit and a power combiner. Beam-forming technology using dynamically phased array has the advantages of simple weight calculation which based on the look directions, high directivity and easy implementation. However, the direction of arrival (DOA) of the desired signal needs to be estimated or known a priori in order to adjust the phase shifters and make the beam main lobe point to the target mobile or base station.
A third example of beam-forming technology is what is used in fully adaptive antenna arrays. The adaptive antenna array system typically comprises multiple (M) antenna elements, M RF units, M down converter to convert RF signals into base band signals, M A/D converters, a weight computation unit to generate the beam-forming weights, and a beam-former. Adaptive antenna array beam-forming technology is performed in base-band by using digital signal processing algorithms and the beam-forming weights are calculated according to weight computing algorithms. Several beam-forming weight computing approaches are described in the paper, “Beam-forming: A Versatile Approach to Spatial Filtering”, IEEE ASSP Magazine, Vol. April, 1988, pp. 4-24. Also, descriptions of beam-forming approaches using adaptive antenna arrays in wireless communication systems is also available in “Application of Antenna Array to Mobile Communications, Part II: Beam-forming and Direction-of-Arrival Considerations” disclosed in Proceeding of IEEE, Vol. 85, No. 8, August 1997, pp. 1195-1245.
Beam-forming with adaptive antenna arrays, yields maximum SINR (Signal-to-Interference plus Noise Ratio) and an adjustable beam pattern, which allows forming the peaks to the desired signal (S) and nulling of interference signals (I).
Such a system is disclosed in U.S. Pat. No. 6,049,307, which features an adaptive phased antenna array using the weight memory unit to adjust the beam directions. This patent features an adaptive phased array, and the beam direction is scanned by adjusting the amplitudes and phases of received RF signals by using a weight memory unit which stores pre-computed weights (amplitudes and phases of RF signals supplied to each antenna element).
For the application of beam-forming technology in wireless communication systems, a technically and economically feasible method is to use switch beam antenna array where the fix-beams are formed by applying phase shift to the individual antenna elements in the antenna array. Generally, in switched beam-forming technology, one of a set of fixed-beams is selected to the desired mobile or base station based on the best measurement of received signal power. This fixed-beam approach could offer feasible coverage and capacity extension especially in a macro cell environment but the performance of this approach will be degraded in large angle spread or multipath propagation environment.